DESCRIPTION: An abundance of evidence has emerged which suggests that the induction of apoptosis may be a critical mechanistic determinant of successful tumor eradication by antineoplastic therapies. This evidence derives largely from studies of cell cycle regulation and the identification of key modulators of cell growth and death. The most commonly aberrant gene in human cancer is p53, a factor clearly important from the triggering of apoptosis by many antitumor treatments. The investigators have extensively studied p53 dependent events which transmit growth control (or death) signal to the cellular machinery. Using genetically defined, in vitro-generated tumor cells, the investigators have demonstrated p53's critical role in mediating apoptosis by the DNA damaging treatments, radiation, and doxorubicin, in vitro and in animal models. p53 loss correlates with sensitivity to apoptosis induction and refractory tumor growth in vivo. On this basis, they have devised a strategy for identifying drugs capable of restoring apoptosis to refractory tumors cells through p53 independent pathways. They found that the vast majority of standard antineoplastic drugs induce p53 dependent apoptosis, potentially explaining the relative ineffectiveness of these agents against so many malignancies (i.e. p53 loss as multi-drug resistance event). This proposal seeks to utilize this system of geneticallydefined tumor cells to further identify p53 independent apoptosis triggers at both the drug and mechanistic levels. They will (1) develop and refine assays for identifying agents eliciting p53 independent apoptosis with tumor cell selectivity, (2) examine cell cycle kinetics activities of which underlie unique p53 independent behavior, (3) determine whether cell cycle regulating drugs and radiation sensitizers are capable of restoring apoptotic death to radioresistant cells in our defined system, and (4) elucidate mechanisms of pan-resistance to apoptosis induction occurring in resistant tumors which still contain wild type p53 (identifying complementation groups of apoptosis regulators). By analyzing results of this work in several different genetically defined and controlled target cell lines (including primary cultures) the broader relevance of identified effects will be examined with the goal of producing therapies which target p53 deficient malignancies.